Tuning UV Absorption in Imine-Linked Covalent Organic Frameworks via Methylation

Covalent organic frameworks (COFs) are porous materials with high surface areas, making them interesting for a large variety of applications including energy storage, gas separation, photocatalysis, and chemical sensing. Structural variation plays an important role in tuning COF properties. Next to the type of the building block core, bonding directionality, and linking chemistry, substitution of building blocks provides another level of synthetic control. Thorough characterization and comparison of various substitution patterns is relevant for the molecular engineering of COFs via rational design. To this end, we have systematically synthesized and characterized multiple combinations of several methylated and non-methylated building blocks to obtain a series of imine-based COFs. This includes the experimental assignment of the COF structure by solid-state NMR. By comparing the properties of all COFs, the following trends were found: (1) upon methylation of the aldehyde nodes, COFs show increased Brunauer–Emmett–Teller surface areas, reduced pore collapse, blue-shifted absorbance spectra, and ∼0.2 eV increases in their optical band gaps. (2) COFs with dimethylated amine linkers show a lower porosity. (3) In tetramethylated amine linkers, the COF porosity even further decreases, the absorbance spectra are clearly red-shifted, and smaller optical band gaps are obtained. Our study shows that methyl substitution patterns on COF building blocks are a handle to control the UV absorbance of the resulting frameworks.


A. Synthetic Procedures
. FT-IR spectra of triplicate Me 3 TFB-Me 2 PA COFs to show the repeatability of the synthesis.
The C=N imine stretch is at 1623 cm -1 .

C. Powder X-Ray Diffraction Analysis
All FT-IR spectra indicate that the synthesis was repeatable which is also confirmed by nitrogen sorption measurements. Therefore, only one representative PXRD and ssNMR spectra of the triplicates will be displayed.
PXRD pattern were simulated in VESTA after optimizing the structures. Two different structures (flat and non-planar) have been computed and the structure with the lowest energy has been chosen as comparison with the experimental PXRD. All COFs containing Me 3 TFB are lowest in energy when the COF structure is non-planar. Those COFs have eclipsed crystal structures. The only exception is TFB-Me 4 PA whose crystal structure has more overlap with the staggered model.      The PXRD diffractograms were measured after re-isolation and drying at 120 °C overnight.

D. 13 C CPMAS NMR Spectra
This chapter is organized showing all ssNMR spectra for each COF.
If one is interested in ssNMR spectra at different contact times, the following spectra are relevant: SI    Figure S42. 13 C CPMAS solid state NMR spectra at 11 kHz and different contact times.

E. Nitrogen Sorption Analysis
All COFs have been synthesized three times and were then divided in two batches for different activation methods: 1. 120 °C, oven-dried, overnight 2. 120 °C, vacuum-oven-dried, overnight For all samples nitrogen sorption analysis was carried out and the BET surface area has been calculated. The range for the linear regression has been the same for all repetitions of the same COF and for both activation methods. The same DFT model has been used for one COF structure to determine the pore size distribution. All graphs displayed are representative for the respective COF.  This COF has been tried to synthesize at least several times of which three samples were repeatable to each other, but in general this COF is not really repeatable in porosity. This is most likely due to the steric hinderance in combination with TFB which forms less stable COFs compared to Me 3 TFB.
The repetition of this COFs have a small decrease in the adsorption isotherm between 0.37-0.93 and 0.25-0.89 p/p 0 while the isotherm in Figure S48 does not show this decrease but a plateau. The surface area calculation is not affected by this decrease, because the used range for fitting is at lower relative pressures.  TFB-Me 4 BD vac. activ.

F. Optical Properties
Samples were prepared by pressing the respective COF powder as a thin film between two glass plates. The samples were measured in an integrating sphere and the diffuse reflectance and diffuse transmission have been recorded. The incoming light beam (100%) gets partly reflected on the sample, partly absorbed by the sample and the rest of the light is transmitted. By detecting reflected and transmitted light, the absorbance can be calculated. 4

G. Tauc plots
To obtain the optical bang gaps, the absorbance, as recorded in the measurement, was transformed into reflectance by neglecting the transmission contribution. The reflectance (R) is converted into the so-called Kubelka-Munk function (Equation 2) to calculate the optical band gap E g . Kubelka and Munk 5 have developed a model for the appearance of paint films which is a continuous model using the two phenomenological parameters K (absorption coefficient) and S (scattering coefficient). Those can be derived from the reflectance by Equation 3 and Equation 4 assuming a direct allowed transition (n=1/2). 6,7 Out of these coefficients F(R) can be calculated by  Coordinates available as separate files. Coordinates available as separate files.